Olefin metathesis catalysis is a powerful technology, which in recent years has received tremendous attention as a versatile method for the formation of carbon-carbon bonds and has numerous applications in organic synthesis and polymer chemistry (R. H. Grubbs, Handbook of Metathesis, Vol. 2 and 3; Wiley VCH, Weinheim, 2003). The family of olefin metathesis reactions includes ring-closing metathesis (RCM), cross metathesis (CM), ring-opening metathesis polymerization (ROMP), and acyclic diene metathesis polymerization (ADMET). The success of olefin metathesis stems from the development of several well-defined transition metal complexes, such as the Schrock molybdenum catalysts and the Grubbs ruthenium and osmium catalysts (see, e.g., Schrock (1999) Tetrahedron 55, 8141-8153; Schrock (1990) Acc. Chem. Res. 23, 158-165; Grubbs et al. (1998) Tetrahedron 54, 4413-4450; Trnka et al. (2001) Acc. Chem. Res. 34, 18-29; Grubbs, Handbook of Metathesis, Vol. 1; Wiley VCH, Weinheim, 2003). Following the discovery of these complexes, a significant amount of olefin metathesis research has focused on tuning the ruthenium and osmium carbene catalysts in order to increase their activity, selectivity, and/or stability. The most common strategy has involved the replacement of mono-dentate ligands with other mono-dentate ligands to provide the catalytic complexes with new and useful properties.
The original breakthrough ruthenium catalysts were primarily bisphosphine complexes of the general formula (PR3)2(X)2M═CHR′ wherein M is ruthenium (Ru) or osmium (Os), X represents a halogen (e.g., Cl, Br, or I), R represents an alkyl, cycloalkyl, or aryl group (e.g., butyl, cyclohexyl, or phenyl), and R′ represents an alkyl, alkenyl, or aryl group (e.g., methyl, CH═C(CH3)2, phenyl, etc.) (see Nguyen et al. (1992) J. Am. Chem. Soc. 1992, 114, 3974-3975; Schwab et al. (1995) Angew. Chem., Int. Ed. 34, 2039-2041; Schwab et al. (1996) J. Am. Chem. Soc. 118, 100-110). Examples of these types of catalysts are described in U.S. Pat. Nos. 5,312,940, 5,969,170 and 6,111,121 to Grubbs et al. While such complexes are capable of catalyzing a considerable number of olefin metathesis transformations, these bisphosphine complexes can exhibit lower activity than desired and, under certain conditions, can have limited lifetimes.
More recent developments in the field have led to greatly increased activity and stability by replacing one of the phosphine ligands with a bulky N-heterocyclic carbene (NHC) ligand (Scholl et al. (1999) Organic Letters 1, 953-956) to give complexes of the general formula (L)(PR3)(X)2Ru═CHR′, wherein L represents an NHC ligand such as 1,3-dimesitylimidazole-2-ylidene (IMes) and 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (sIMes), X represents a halogen (e.g., Cl, Br, or I), R represents an alkyl, cycloalkyl, or aryl group (e.g., butyl, cyclohexyl, or phenyl), and R′ represents an alkyl, alkenyl, or aryl group (e.g., methyl, CH═C(CH3)2, phenyl, etc.). Representative structures include complex A (ibid.), complex B (Garber et al. (2000) J. Am. Chem. Soc. 122, 8168-8179), and complex C (Sanford et al. (2001) Organometallics 20, 5314-5318; Love et al. (2002) Angew. Chem., Int. Ed. 41, 4035-4037):

Unlike prior bisphosphine complexes, the various imidazolylidine catalysts effect the efficient formation of trisubstituted and tetrasubstituted olefins through catalytic metathesis. Examples of these types of catalysts are described in PCT publications WO 99/51344 and WO 00/71554. Further examples of the synthesis and reactivity of some of these active ruthenium complexes are reported by Fürstner et al. (2001) Chem. Eur. J. 7, No. 15, 3236-3253; Blackwell et al. (2000) J. Am. Chem. Soc. 122, 58-71; Chatterjee et al. (2000) J. Am. Chem. Soc. 122, 3783-3784; Chatterjee et al. (2000) Angew. Chem. Int. Ed. 41, 3171-3174; Chatterjee et al. (2003) J. Am. Chem. Soc. 125, 11360-11370. Further tuning of these catalysts led to even higher activity by using bulkier imidazolylidine ligands such as 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidenes (Dinger et al. (2002) Adv. Synth. Catal. 344, 671-677) or electron deficient phosphine ligands such as fluorinated aryl phosphines (Love et al. (2003) J. Am. Chem. Soc. 125, 10103-10109).
Another example of ligand substitution that has led to enhanced catalyst activity is the replacement of the phosphine ligand in the (L)(PR3)(X)2M═CHR′ complexes with one or two pyridine-type ligands to give compounds of the general formula (L)(L′)n(X)2M═CHR′ wherein n=1 or 2, L represents an imidazolylidine ligand, L′ represents a pyridine (Py) or substituted pyridine ligand, X represents a halogen (e.g., Cl, Br, or I), and R′ represents an alkyl, alkenyl, or aryl group (e.g., methyl, CH═C(CH3)2, phenyl, etc.). These pyridine complexes are extremely fast-initiating and catalyze living ring-opening metathesis polymerizations (Choi et al. (2003) Chem. Int. Ed. 42, 1743-1746) as well as highly challenging processes such as olefin cross metathesis with acrylonitrile (Love et al. (2002) Angew. Chem. Mt. Ed. 41, 4035-4037).
Yet another example of mono-dentate ligand substitution is the replacement of the halogen ligands with aryl-oxo ligands, which in one example has led to a catalyst with enhanced activity: (L)(L′)n(RO)2Ru═CHR′ wherein n=1, L represents an imidazolylidine ligand, L′ represents a pyridine ligand, R represents a fluorinated aryl group, and R′ represents an alkyl, alkenyl, or aryl group (Conrad et al. (2003) Organometallics 22, 3634-3636).
A different strategy to tune olefin metathesis catalysts involves linking two of the ligands that are attached to the metal center. Of particular interest are the chelating carbene species reported by Hoveyda and others (Gaber et al. (2000) J. Am. Chem. Soc. 122, 8168-8179; Kingsbury et al. (1999) J. Am. Chem. Soc. 121, 791-799; Harrity et al. (1997) J. Am. Chem. Soc. 119, 1488-1489; Harrity et al. (1998) J. Am. Chem. Soc. 120, 2343-2351). These catalysts are exceptionally stable and can be purified by column chromatography in air. Representative such catalysts, designated Catalyst PR-1 and PR-2, are illustrated in FIG. 1. Catalyst PR-2 combines excellent stability and enhanced activity, and actively promotes the cross-metathesis of acrylonitrile and terminal olefins in moderate to excellent yields.
While most of these efforts have focused on increasing the activity and initiation rate of the ruthenium carbene metathesis catalysts, there remains a need for highly active catalysts that initiate more slowly (i.e., are more latent). This can be a particularly beneficial feature when performing metathesis polymerizations, which, in practice, typically require a significant period of time (the “work-time”) within which to mix, handle, and process the catalyst/resin mixture before it gels or solidifies. For one-part catalyst systems, such as the metal carbene olefin metathesis catalysts, latency is generally achieved through temperature variation. Either the catalyst/resin mixture can be handled at a low enough temperature to sufficiently inhibit polymerization or the catalyst must be designed to be heat-activated to allow sufficient work-time at ambient temperatures.
One example of a thermally activated, latent metathesis polymerization catalyst system utilizing slower initiating ruthenium and osmium vinylidene complexes was described in U.S. Pat. No. 6,107,420. However, only a modest degree of control of the latency can be achieved by varying the identity of the substituent groups of the vinylidene ligand and such vinylidene complexes are often not efficient metathesis catalysts for unstrained olefins. Another example of a latent olefin metathesis catalyst that contains a chelating carbene ligand is the 2-pyridylethanyl ruthenium carbene complex (PR3)(Cl)2Ru(CH(CH2)2—C,N-2-C5H4N) by reacting a (PR3)2(Cl)2Ru′CHR′ complex with 2-(3-butenyl)pyridine developed by van der Schaaf (van der Schaaf et al. (2000) J. Organometallic Chemistry 606, 65-74). These types of catalysts are also described in U.S. Pat. No. 6,306,987. Although these catalysts do initiate more slowly than their bis-phosphine counterparts, they lack the high activity of the NHC catalyst systems. A further type of latent olefin metathesis catalysts is described by van der Schaaf in U.S. Pat. No. 6,077,805. These latter catalysts are hexacoordinate ruthenium or osmium complexes wherein two of the six ligands are preferably pyridine ligands.
In trying to develop new examples of latent, high-activity catalysts containing NHC ligands, the teaching in the prior art provides no clear direction. U.S. Pat. No. 6,077,805 teaches that hexacoordinate phosphine-ligated complexes of the general structure (PR3)(X)2(L)2M=CHR′, wherein the L ligands are pyridines or substituted pyridines or together are chelating bipyridines, are latent metathesis catalysts. Data presented in U.S. Patent Application Publication Number 2002/0177710 confirm the latency of such catalysts but also show that, in contrast, related hexacoordinate NHC-ligated complexes of the general structure (NHC)(X)2(L)2M=CHR′ are not latent catalysts but, in fact, are actually some of the most rapidly initiating catalysts of this type ever observed (e.g., cf. Choi et al. (2003) Angew. Chem. Int. Ed. 42, 1743-1746 and Love et al. (2002 Angew. Chem. Int. Ed. 41, 4035-4037). U.S. Pat. No. 6,306,987 teaches that phosphine-ligated bridging carbene complexes of the general structure D are latent metathesis catalysts, whereas similar NHC-ligated complexes of the general structure E are not (e.g., Courchay et al. (2003) Macromolecules 36, 8231-8239). These observations suggest that it is difficult to achieve latency with the high-activity catalysts comprising NHC ligands.

Accordingly, despite advances in the art, there is a continuing need for olefin metathesis catalysts that initiate slowly while maintaining the high activity associated with NHC-based catalysts.